ABSTRACT
Generally speaking, there are three types of ¢ts between mating features of size on mating parts. These are clearance ‚ts, transition ‚ts and interference ‚ts. These are standard ¢t classi¢cations; each is based on how mating features on mating parts interact. U.S. and international standards de¢ne systems of limits and ¢ts that govern these ¢t classi¢cations, such as ASME and ISO standards. Information on these standard systems of limits and ¢ts can be found in the Machinery’s Handbook or in documents from the applicable standards governing bodies. Fit classes or grades may be designated using numeric values or using codes. In ISO 286-2:1988 and ASME B4.2-1978 (R2004), ¢ts are designated using codes representing the tolerance grade or ¢t class. Different codes are used in the standards, but the standards essentially provide very similar information. Charts are consulted in these standards to determine the required size limits for the mating features. The nominal (or basic*) sizes are stated on the drawing followed by the applicable code. Alternatively, the equivalent tolerances or limits may be speci¢ed.
